The present invention relates generally to thermal mass flow sensors, and more particularly to a differential thermal mass flow sensor which is designed to provide a constant temperature gradient in the transfer of heat to and from a fluid flowing through the sensor.
The mass flow rate of a fluid is known to be proportional to the amount of heat required to elevate the fluid temperature by a fixed amount as it flows through a laminar flow channel. In a differential thermal flow sensor, the fluid passes through a conduit in which a symmetrically increasing and decreasing temperature distribution is maintained, and a comparison is made between heat transfer to and from the fluid to determine the flow rate of the fluid in the conduit.
Sensors which operate on the basis of this principle generate a flow signal that is proportional to the mass flow rate. They also generate a small noise signal that is random in character and slowly fluctuating in time. For a simple sensing conduit (with no bypass), the amplitude of the noise signal is essentially independent of flow, yet dependent on the thermal properties of the fluid flowing in the conduit. Because the noise signal is present even at zero flow, it is referred to as xe2x80x9czero-flow noisexe2x80x9d. The zero-flow noise limits the instantaneous flow resolution of the thermal flow sensor over its entire flow sensing range. It also defines the minimum flow that can be effectively measured using a particular sensor. Finally, the presence of a significant, slowly fluctuating zero-flow noise signal hinders the calibration (particularly the zeroing) of instruments employing thermal flow sensors.
The zero-flow noise of these thermal flow sensors appears to be caused by a slow thermal churning or amplitude-limited thermal instability of a fluid in contact with (primarily within) the sensor conduit. For conventional sensors that operate by detecting relative resistance changes caused by small flow-induced temperature differentials on resistive heaters located symmetrically on the inlet and outlet halves of the sensor conduit, the amplitude of the zero-flow noise signal is several orders of magnitude larger than the well-known Johnson noise expected from such resistive heaters at the operating temperature of the sensor in the range of frequency bandwidths commonly employed.
Because some sensor designs are intrinsically less noisy than others, it would be advantageous to design a sensor which reduces the sensor zero-flow noise relative to the sensor flow signals of flow sensors known in the art.
As the zero-flow temperature fluid profile along the sensor conduit approaches the shape of a symmetrical (isosceles) triangle, the zero-flow sensor noise decreases dramatically relative to the flow-induced sensor signal. This may be a result of the insignificance or absence of non-uniform heat conduction along the heated flow path (term involving the second derivative of temperature with respect to position along the sensor length) in heat conduction equations applied to these sensors in comparison to convection along the heated flow path (term involving the first derivative of temperature with respect to position along the sensor length). For a sensor with a triangular temperature distribution profile, all such second derivative terms are zero everywhere along the thermally active conduit length except at the apex of the triangular temperature distribution.
The tapered heater sensor disclosed in U.S. Pat. No. 5,693,880 to Maginnis, Jr., assigned to the assignee of the present invention (hereafter, the ""880 patent), provides an approximately triangular temperature distribution profile away from the center of the heated flow conduit, with a blunted apex at the center of the heated conduit that can extend over a significant portion of the conduit length, as shown in the graph of FIG. 2. The present invention describes a heat source arrangement which provides a nearly perfectly symmetrical triangular temperature distribution profile at zero flow with a sharply pointed apex, as shown in the graph of FIG. 5.
The term xe2x80x9cheater densityxe2x80x9d, as used herein, refers to the heating power applied to a heated flow passageway per unit length of the passageway in a thermal flow sensor. Heater density is a continuous function of position along the heated portion of the flow passageway. It is known that a mass flow sensor which is characterized by a rectangular heater density function, in which the heating power is constant over the heated length of conduit, will exhibit nonlinearities in the temperature distribution profile. Such nonlinearities are disadvantageous, as they are associated with relatively high noise, reduced sensitivity, non-uniform heat transfer to and from the flowing fluid, and a reduced range of flow rates that are measurable with the sensor.
The ""880 patent discloses a differential thermal mass flow sensor that provides a non-uniform heater density over the heated portion of the flow sensor conduit. The flow sensor conduit may be heated, for example, with a pair of resistive wire conductors which are wound nonuniformly around a conventional cylindrical sensor tube so that the windings are most closely spaced at the junction of the two conductors, and most widely spaced at the outer ends of the conductors. This nonuniform winding configuration provides a tapered, or uniformly varying, heater density, wherein the heating power is nonuniform over the length of the winding.
In the flow sensor of the ""880 patent, the temperature of the fluid varies approximately linearly as a function of position at some distance from the junction of the conductors, and nonlinearly in the region at the junction of the conductors. This temperature distribution profile is shown in FIG. 2 as a triangle having a flattened apex. The flattened apex portion of the temperature distribution function is caused by insufficient heating power at the junction of the conductors and is associated with reduced sensitivity, increased noise due to thermal instability, and generally decreased flow sensor performance relative to the performance expected from a more nearly ideal sensor with a triangular temperature distribution profile which extends over nearly the full length of the sensor.
Accordingly, it would be advantageous to provide a thermal mass flow sensor which can provide more nearly ideal heat transfer to and from a flowing fluid without inducing these and other deficiencies.
According to one aspect of the invention, there is provided a thermal mass flow sensor comprising a flow sensor conduit adapted for fluid flow therethrough, a first heating element disposed proximal to the sensor conduit and configured to establish a distributed, uniformly varying heater density along a portion of a heated length of the sensor conduit, and a second heating element disposed proximal to the sensor conduit and to the first heating element and configured to establish a concentrated heater density at a preselected location within the heated length of the sensor conduit. The temperature distribution function of a fluid within the heated length of the sensor conduit is a substantially symmetrical triangular distribution function.
The first heating element preferably comprises a pair of distributed heat sources. The second heating element preferably comprises a single concentrated heat source which is disposed between the two distributed heat sources. In a preferred embodiment, the distributed heat sources are symmetrically disposed on the sensor conduit with respect to the concentrated heat source, which is located between them, preferably at or near the midpoint of the heated length of the sensor conduit.
The distributed heater density in the flow sensor of the invention preferably increases uniformly from substantially equal minimum values near the ends of the heated length of the sensor conduit toward the center of the heated length of the conduit, and then spikes to a substantially greater value at a point corresponding to the centrally located concentrated heat source. The preferred minimum value of the distributed heater density is zero, or as close to zero as is practical for sensor manufacture, and occurs at the outer ends of the heated portion of the sensor conduit.
In a preferred embodiment, the distributed heat sources provide minimum heat at the ends of the heated length of the sensor conduit and maximum heat at or near the midpoint of the heated length of the conduit. In one preferred embodiment, the distributed heat sources comprise resistive heat sources, such as, for example, resistive coils wound around the a conventional cylindrical tube, or resistive films deposited onto such a sensor tube. In another embodiment, the distributed heat sources comprise radiant heat sources, such as, for example, optical fiber arrays which are disposed about the flow sensor conduit in such a manner as to direct radiant energy onto the conduit and produce the desired triangular temperature distribution along the heated length of the conduit.
If resistive coils or radiant heating optical fibers are employed as the distributed heat sources, the spacing of adjacent windings of the coils or ends of the fibers preferably varies continuously with position on the sensor conduit so as to achieve the desired temperature distribution profile. If resistive films are employed as the distributed heat sources, the thickness and/or width of the deposited films preferably varies continuously with position along the sensor conduit in order to achieve the desired temperature distribution profile. For example, greater heat is provided from a closely wound coil and from a thinner and/or narrower resistive film. Therefore, for maximum heating near the center of the heated length of the tube, coils are wound more closely and resistive films are thinnest and narrowest. Conversely, less heat is provided from a coil having widely spaced turns and from thicker and/or wider resistive films. Therefore, for minimum heating near the ends of the heated length of the tube, coils are more loosely wound, and resistive films are thickest and/or widest.
The thermal mass flow sensor further includes connections for providing an electrical current to the first and second heating systems.
The first heating system preferably functions also as a differential thermal sensor and is adapted to generate a signal representative of fluid temperature differential at symmetric locations on the upstream and downstream portions of the heated length of the conduit, and thus of the fluid flow rate through the sensor conduit. The temperature sensing function may also be separated from the heating function used to establish the triangular temperature distribution by providing additional uniform resistive windings or films, electrically insulated from the primary heating elements, on the upstream and downstream portions of the heated length of the sensor conduit. They may be operated at low power dissipation so as to produce negligible heating themselves. In the embodiment employing end radiation from a plurality of optical fibers to establish a triangular temperature profile, it may also be desirable to employ electrically resistive elements (windings or films) as passive temperature sensors to sense flow-induced temperature changes along the sensor conduit. All such variations and substitutions of alternate differential temperature sensing elements applied to different portions of the flow conduit are considered to fall within the scope and claims of this invention, provided that the zero-flow temperature distribution along the flow conduit is approximately symmetrically triangular, as described above.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.